The present invention relates to a process for compensating errors of an acceleration sensor and, more particularly, to such a process for use in a vehicle for measurement of an acceleration in a defined direction.
A process for compensating acceleration sensor errors is generally known as shown in DE 3,705,932 A1. According to this process, a longitudinal-acceleration sensor is checked to determine if it is functioning and suitably calibrated when the vehicle is travelling without any wheel slip. A longitudinal acceleration of the vehicle can therefore be derived from the wheel speeds.
A disadvantage of this known process is that calibration and, if appropriate, a detection of a functional fault of an acceleration sensor is possible only when the direction of the acceleration measured by the acceleration sensor coincides with the direction of the longitudinal acceleration of the vehicle. A functional check of a transverse-acceleration sensor is therefore impossible.
Furthermore, DE 3,702,824 shows it is known to eliminate the influence of the gravitational acceleration g as a result of the occurrence of rolling angles of the vehicle in the determination of the transverse acceleration by arranging two acceleration sensors at a specific angle to one another and by evaluating their signals.
A disadvantage of this process is that two acceleration sensors have to be used in order to eliminate the influence of gravitational acceleration on the acceleration to be determined. Moreover, the acceleration sensors have to be arranged at specific angles to one another, that is to say inaccuracies in respect of these angles cannot be detected in this known process.
It is likewise known from DE 3,627,241 A1 to check the functional capacity of an acceleration sensor by applying a test signal to the acceleration sensor and by comparing the output signal thereby obtained with a desired trend for this output signal.
A disadvantage of this test signal process is that only the electrical transmission of the acceleration sensor can be checked by a test signal. If, for example on a seismic acceleration sensor, a fault occurs as a result of catching of the mass to be accelerated, this fault cannot be detected by applying a test signal. In carrying out this process, it is at most conceivable to cause a specific force to act on the mass, so that a functional check of the acceleration sensor can thus be conducted by comparing the output signal obtained from the acceleration sensor with the desired value of the output signal of the acceleration sensor which should be obtained in response to an acceleration corresponding to the specific force. However, the disadvantage of such a process is that functional check would involve a relatively high outlay.
An object of the present invention is to detect a functional fault of a defective acceleration sensor and/or to detect and compensate differences, occurring during installation or after a lengthy operating period, between the direction of the acceleration to be measured in the direction R.sub.def and the direction of the acceleration which is sensed by the sensor.
According to the present invention, in a process for compensating errors of an acceleration sensor, this object is achieved by a process in which part of an output signal A.sub.B of the acceleration sensor, based on the acceleration a.sub.dif1 in a direction R.sub.dif1 differing from a defined direction R.sub.def of the acceleration a.sub.def to be measured, is detected by subjecting the output signal A.sub.B of the acceleration sensor to a sliding average method when it is concluded from general driving conditions of the vehicle that the acceleration a.sub.def in the defined direction R.sub.def must momentarily be equal to zero. A signal S.sub.B of the acceleration sensor is generated from the output signal A.sub.B of the acceleration sensor by subtracting the result RES of the sliding average method from the output signal A.sub.B of the acceleration sensor, in order to arrive at the signal S.sub.B of the acceleration sensor representing the acceleration a .sub.def in the defined direction R.sub.def. A functional check of the acceleration sensor can be conducted by basing at least part of the output signal A.sub.B of the acceleration sensor on an acceleration a.sub.dif2 in a direction R.sub.dif2 which is determined by at least one further device for the determination of the acceleration a.sub.dif2 in this direction R.sub.dif2. The output signal A.sub.B or, if appropriate, the signal S.sub.B of the acceleration sensor is compared with the determined value of the acceleration a.sub.dif2 in this direction R.sub.dif2, and a functional fault of the acceleration sensor is concluded from a deviation when it is derived from general driving situations that only an acceleration a.sub.dif2 in this direction R.sub.dif2 acts on the acceleration sensor.
Advantages of the present invention over conventional processes are that no further acceleration sensor is required to compensate for directional errors of the acceleration sensor. The detection of a functional fault of the acceleration sensor is now possible in an especially advantageous way in a vehicle with an anti-lock system (ABS) and/or a drive-slip control (ASR). In these systems, a value for the vehicle acceleration is determined from the wheel speeds. This value can then be used advantageously for the functional check of the acceleration sensor. If deviations of the determined longitudinal accelerations of the vehicle between the value of the acceleration sensor and the value determined from the wheel speeds are found, it can be concluded with high probability that there is a functional fault of the acceleration sensor by virtue of the redundancy of the ABS or ASR signals provided for safety purposes.
In order to determine the acceleration of a vehicle in a particular direction, an acceleration sensor is mounted in such a way that it measures the acceleration a.sub.def in the defined direction R.sub.def. If this direction R.sub.def of the acceleration a.sub.def to be measured is in the horizontal plane, the output signal A.sub.B of the acceleration sensor can differ from the expected output signal A.sub.B in response to the acceleration a.sub.def to be measured, when the direction R.sub.def of the acceleration a.sub.def measured by the acceleration sensor is inclined at an angle .beta. relative to the horizontal plane. The deviation of the output signal A.sub.B from the expected output signal A.sub.B is based on the gravitational acceleration g in the vertical direction which contributes the acceleration B.sub.ver of (g*sin.beta.) to the output signal A.sub.B. This contribution B.sub.ver in the output signal A.sub.B is always present; that is to say, when the vehicle is travelling without acceleration, this contribution B.sub.ver makes up the entire output signal A.sub.B, whereas, when the vehicle is travelling with acceleration, part of the output signal A.sub.B is based on the accelerated travel of the vehicle, and another part of the output signal A.sub.B is based on the contribution B.sub.ver.
On the other hand, this angle .beta. can also come about because the acceleration sensor is installed inexactly in its position, and, on the other hand, the acceleration sensor can change its position during operation. Moreover, the angle .beta..noteq.0 can occur when the vehicle is on a stretch with a gradient or a lateral inclination.
In principle, as regards an individual output signal A.sub.B of the acceleration sensor in an unaccelerated driving state of the vehicle, it is not possible to distinguish whether an angle .beta..noteq.0 comes about because the acceleration sensor is inclined relative to the horizontal plane of the vehicle or because the vehicle and, therefore, also the acceleration sensor have an inclination relative to the horizontal plane. In the process according to the present invention for compensating errors of an acceleration sensor the output signals A.sub.B of the acceleration sensor in an unaccelerated driving state are therefore subjected to a sliding average method, in order to filter brief fluctuations of the output signal A.sub.B of the acceleration sensor in the unaccelerated driving state (based on a gradient or lateral road inclination) and to obtain the fraction of the output signal A.sub.B of the acceleration sensor which is based on an inclination of the acceleration sensor relative to the horizontal plane of the vehicle. Subtraction of the result RES of the sliding average method from the momentarily measured acceleration a.sub.def of the acceleration sensor then produces the momentary acceleration in the defined direction R.sub.def in relation to the horizontal plane of the vehicle.
In order to avoid fluctuations possibly occurring in the result RES of the sliding average method, it is possible for output signals A.sub.B of the acceleration sensor not to be included in the sliding average method, even if there is an unaccelerated driving state, when the output signals A.sub.B have a difference greater than a predetermined threshold value SW in relation to the result RES of the sliding average method. With an increasing gradient, the acceleration a.sub.def measured by the acceleration sensor increases in an unaccelerated driving state. When a threshold value SW for taking into account the output signals of the acceleration sensor is used in carrying out the sliding average method, gradients or lateral inclinations of the road therefore remain neglected above a threshold S obtained from this threshold value SW. The threshold value SW is appropriately selected to be of such an order of magnitude that the threshold value SW is exceeded above a gradient or lateral road inclination of approximately 1.degree. to 2.degree..
It is also possible to cancel the process according to the present invention for a specific period of time when the vehicle is travelling along a stretch with a gradient above the threshold value SW. The gradient of the stretch covered by the vehicle can be obtained in a way known per se from the engine torque of the vehicle and from the transmission ratio of the engine speed to the wheel speed.
Since a gradient or lateral inclination of the road influences the output signal A.sub.B of the acceleration sensor, it is advantageous to take these factors into account when they have only slight momentary fluctuations. For example, when the vehicle is travelling along a road through a pass, it covers a relatively long stretch uphill or downhill. If the process according to the invention is carried out with the use of a threshold value SW for taking into account output signals A.sub.B of the acceleration sensor, there can therefore be no updating of the value of the sliding average method during the uphill or downhill travel, as long as the road inclination is above the threshold S leading to an output signal A.sub.B of the acceleration sensor which has a difference above the threshold value from the momentarily valid result RES of the sliding average method.
A gradient or a lateral inclination of the road can be taken into account by also taking into account in a sliding average method, in an unaccelerated driving state, output signals A.sub.B of the acceleration sensor, of which the difference from the result RES of the sliding average method is above the threshold value SW. To avoid taking into account output signals A.sub.B of the acceleration sensor, of which the difference from the result RES of the sliding average method is above the threshold value SW and which are based on only brief fluctuations, output signals A.sub.B of the acceleration sensor, of which the difference from the result RES of the sliding average method is above the threshold value SW, are taken into account only when these output signals A.sub.B occur uninterruptedly in a specific frequency Z.sub.max. When, in carrying out the process according to the invention, it is ascertained from the frequency Z of the occurring output signals A.sub.B of the acceleration sensor, of which the difference from the result RES of the sliding average method is above the threshold value SW, that the vehicle is on a stretch of virtually constant gradient or inclination, a more rapid adaptation of the result of the sliding average method can be obtained if the output signals A.sub.B of the acceleration sensor are included in the sliding average method with an increasing weighting. The increase in the weighting of the output signals A.sub.B of the acceleration sensor when the sliding average method is carried out can here take place linearly or progressively.
The state of unaccelerated travel of the vehicle can be derived in an especially advantageous way when the vehicle speed V.sub.F is equal to zero and the output signal of the acceleration sensor has no time change.
If a further device for the determination of the acceleration a.sub.dif2 in a direction R.sub.dif2 is present on the vehicle, a functional check of the acceleration sensor is possible when the projection of the acceleration a.sub.def in the defined direction R.sub.def, measured by the acceleration sensor, onto the direction R.sub.dif2 is not equal to zero and when the acceleration a.sub.def in the direction R.sub.def, as measured by the acceleration sensor, has no acceleration in a direction not equal to R.sub.dif2. Thus, the functioning of a transverse-acceleration sensor can be checked in an especially advantageous way when an anti-lock system (ABS) and/or a drive-slip control (ASR) are present on the vehicle. The vehicle acceleration a.sub.1 in the longitudinal direction of the vehicle, corresponding to the direction R.sub.dif2, is determined by the ABS or the ASR. When the transverse acceleration sensor is now rotated through a specific angle .mu., e.g. about 30.degree., out of the direction transverse relative to the longitudinal axis of the vehicle into the direction of the longitudinal axis of the vehicle, the output signal A.sub.B of the acceleration sensor, representing the acceleration a.sub.def in the defined direction R.sub.def, is determined by the longitudinal acceleration of the vehicle a.sub.a, corresponding to the acceleration in the direction R.sub.dif2, and the transverse acceleration of the vehicle a.sub.q, corresponding to the acceleration in the direction R.sub.dif3, according to the formula: EQU a.sub.def =a.sub.1 * sin (.mu.)+a.sub.q * cos(.mu.)
Thus, the transverse acceleration of the vehicle a.sub.q is obtained from the vehicle speed V.sub.F and the radius of curvature R of the bend as: ##EQU1##
When it is concluded from the general driving situation that there is no transverse acceleration of the vehicle a.sub.q, that is to say the vehicle is not travelling round a bend, that output signal of the acceleration sensor representing the acceleration a.sub.def is produced solely by the longitudinal acceleration of the vehicle a.sub.1. This means that the longitudinal acceleration of the vehicle a.sub.1 determined by the ABS or by the ASR must be equal to the acceleration a.sub.def measured by the acceleration sensor, divided by the factor sin(.mu.). The transverse acceleration of the vehicle a.sub.q is then obtained by substituting the above equation as: ##EQU2##
The process according to the present invention for a functional check of the acceleration sensor is carried out when the vehicle is travelling in a straight line. Travel in a straight line can be detected from the steering angle of the steered wheels of the vehicle or from the steering-wheel angle of the vehicle. The process according to the invention is carried out when these angles are equal to zero.
Alternatively, a travel of the vehicle on a bend can be detected by a comparison of the time derivations of the determined longitudinal acceleration of the vehicle a.sub.1 and of the measured acceleration a.sub.def. If the vehicle is travelling on a bend, with a constant radius of curvature R of the bend, the transverse acceleration of the vehicle a.sub.q changes as a result of the changing vehicle speed v.sub.F. The time change A.sub.3 of the transverse acceleration of the vehicle a.sub.q therefore depends on the vehicle speed v.sub.F and on the radius of curvature R of the bend, according to the relation: ##EQU3## R' being the time derivation of the radius of curvature R of the bend. In straight line travel, the time change of the longitudinal acceleration of the vehicle a.sub.1 is directly proportional to the time change of the measured acceleration a.sub.def, The proportionality constant is here predetermined at sin(.mu.) by the geometry of the arrangement of the acceleration sensor. During travel on a bend, with a time change of the longitudinal acceleration of the vehicle a.sub.1, there is a time change of the transverse acceleration a.sub.q which, in general, is not proportional to the longitudinal acceleration of the vehicle a.sub.1. This proportionality is present only when the time change R' of the radius of curvature R of the bend disappears. In this instance however, the proportionality factor still depends on the vehicle speed v.sub.F and on the radius of curvature R of the bend.
This affords the possibility of detecting a travel on a bend by determining and storing the quotient Q.sub.str, resulting from the geometry, from the time change a.sub.1 and the time change A.sub.2 during travel in a straight line. This quotient Q.sub.str is compared with the particular quotient Q.sub.cur determined from the current operating conditions when at least one of the time changes of the longitudinal acceleration of the vehicle a.sub.1 and/or of the measures acceleration a.sub.def in the defined direction R.sub.def does not disappear. Travel on a bend is concluded from a deviation of the quotient Q.sub.str from the quotient Q.sub.cur.